Electric vehicles, which do not use oil and thereby do not emit CO2, and plug-in hybrid electric vehicles, which have electric motors and an internal-combustion engine and thereby can significantly reduce emission of CO2, have been implemented. Hereinafter, the electric vehicles and plug-in hybrid electric vehicles are generally referred to as “EVs.” As EVs become widespread, the emission of CO2 can be reduced. Thus, the widespread use of EVs can become one measure to prevent global warming.
On the other hand, as EVs become widespread, the amount of electric power, which is used to charge batteries that are power sources of EVs, increases. In other words, the demand for electric power which is used to charge batteries in EVs increases (hereinafter this demand is referred to as “charge electric power demand.” This means that an increase in charge electric power demand will negatively influence the demand-supply balance that has been stabley controlled.
In addition, as a method that solves the global warming issue, a lot of renewable electric power sources, such as photo voltaic generation and wind farm, tend to be introduced.
The output electric powers of the renewable electric power sources fluctuate depending on weather phenomena that are difficult to forecast. Thus, if the number of renewable electric power sources that are introduced increases, it becomes difficult to control the electric power supply. As a result, an increase in renewable electric power sources adversely affects the demand-supply balance of the electric power grid that has been stably controlled.
The use of a number of thermal generators, which can adjust their outputs, in order to deal with both output fluctuations of renewable electric power sources (fluctuations of electric power supply) and an increase in charge electric power demand (demand fluctuations of electric power that occur as EVs become widespread), run counter to the goal of eliminating CO2.
Smart grid technology based on the ICT (Information and Communication Technology) can manage, to some extent, increases and decreases in the demand for electric power that depends on loads used by the users that, up to now, have not been subject to regulation. Thus, the idea, in which the loads that used by the users are used to stabilize the demand-supply balance of the electric power grid, has gained attention in recent years (refer to Patent Literature 1).
As described above, if EVs become widespread, the amount of electric power that is used to charge the batteries of EVs (electric power demand) becomes large. If EVs are permitted to discharge power back to the electric power grid, the amount of electric power discharged from the batteries of EVs (generated electric power) becomes large. Thus, the batteries of EVs are highly useful as controllable loads and power generators. In such situations, technical studies for EVs, such as V2G (Vehicle-to-Grid) and G2V (Grid-to-Vehicle) as described in Non-Patent Literatures 1 and 2, have been carried out.
On the other hand, since EVs are moving means, the batteries of EVs are not always connected to the electric power grid. Thus, it is uncertain when the batteries will be connected to the electric power grid since this depends on the behavior of individual owners of EVs. Consequently, the batteries of EVs need to be managed based on a charging/discharging planning and scheduling that are different from those of stationary batteries and devices such as typical heat pumps that are used to shift the demand at night.
Thus, in a charge system that charges a plurality of EVs by connecting EVs to the electric power grid, taking into consideration the fact that connections between individual EVs and the electric power grid vary over time, charging of individual EVs needs to be controlled in real time and risks to the owners of the EVs (for example, insufficient charge at departure time or seriously deteriorated battery of EV) need to be eliminated. In addition, the influence of EVs upon demand and supply for electric power in the electric power grid needs to be considered on the basis of charging and discharging of EVs connected to the electric power grid. It is difficult, however to find a balance that satisfies the needs of individual owners of EVs for convenience and the need to manage charging and discharging of EVs.
Since batteries have a limited number of charge/discharge cycles and are expensive, it is difficult to imagine that electric power of the battery of each EV will be used for purposes other than powering the EV, except in cases of emergency. With regard to this matter, an example of charge control for batteries of EVs will be described in the following. However, from the point of view of how to effectively use the period during which batteries are connected to the electric power grid the discharge control for batteries of EVs is the same as their charge control.
FIG. 1 is a schematic diagram describing the significance of controlling the total charge electric power of EVs. FIG. 1 simply shows an example of surplus night time electric power 1 and surplus photo voltaic electric power 2.
In FIG. 1, base source electric power 3 is electric power supplied from an electric power source whose output is difficult to adjust, namely electric power supplied from an electric power source in which the capability to make adjustments to handle fluctuations in electric power is low.
Electric power demand 4 is electric power that is consumed by users and fluctuates depending on various conditions such as seasons and holidays. Electric power demand 4 fluctuates depending on human activities, and the Electric power demand period is a 24 hours cycle. Electric power demand 4 tends to decrease in the middle of the night.
If electric power demand 4 that is low at night becomes lower than base source electric power 3 at night, since the adjustment capability of the electric power source (base power source) of base source electric power 3 is low, a surplus of electric power 1 occurs at night.
Surplus night time electric power 1 is currently used in such a manner that a hydraulic power generator pumps up water and stores surplus night time electric power 1 as potential energy, or a device such as a heat pump that stores and consumes surplus night time electric power 1 is used at a lowered night time electric power fare.
In FIG. 1, in the daytime, electric power that is generated by sunlight (photo voltaic source electric power) 5 is large, and the sum of base source electric power 3 and photo voltaic source electric power 5 (hereinafter the sum is referred to as “sum of electric power”) becomes greater than electric power demand 4.
If the sum of electric power is greater than electric power demand 4, the electric power, which is generated by subtracting electric power demand 4 from the sum of electric power, becomes surplus like surplus night time electric power 1. In FIG. 1, the surplus is represented as surplus photo voltaic electric power 2.
As long as the owner's EV has been fully charged by the departure time, he or she does not care about what electric power source his or her EV uses. As a result, if users are offered an incentive to lower the charge electric power costs and they can use it, their EVs may be used as devices that use the surplus electric power in a time range for which the foregoing electric power becomes surplus.
Thus, if EVs become widespread, pump-up water typed hydraulic power generators, which consume surplus night time electric power, and expensive electric power generators, which have high adjustment capability and decrease surplus photo voltaic electric power, can be eliminated.
Non-Patent Literature 3 describes a charge control method for fleet EVs as a method that controls the total charge electric power of a plurality of EVs and charging of individual EVs. The technique described in Non-Patent Literature 3 assigns charge electric power to a plurality of EVs after evening until morning such that the charged electric power does not exceed the capacity of an electric power reception system including charging devices. Charge electric power is assigned to EVs in such a manner that the charge emergencies of individual EVs are calculated on the basis of the remaining times until the departure time and are calculated on the basis of required charge electric power amount for individual EVs. Thereafter, EVs to be charged are selected in the order of the charge emergencies such that the total amount of charge electric power amounts do not exceed the capacity.
Non-Patent Literature 4 describes an interface through which charging of EVs is controlled.